254 Influence of environment on animals 
(ce) On the production of twins from one egg through a change 
in the chemical constitution of the sea-water. 
The reader is probably familiar with the fact that there exist two 
different types of human twins. In the one type the twins differ as 
. Inuch as two children of the same parents born at different periods ; 
they may or may not have the same sex. In the second type the 
twins have invariably the same sex and resemble each other most 
closely. Twins of the latter type are produced from the same egg, 
while twins of the former type are produced from two different eggs. 
The experiments of Driesch and others have taught us that twins 
originate from one egg in this manner, namely, that the first two cells 
into which the egg divides after fertilisation become separated from 
each other. This separation can be brought about by a change in the 
chemical constitution of the sea-water. Herbst observed that if the 
fertilised eggs of the sea-urchin are put into sea-water which is freed 
from calcium, the cells into which the egg divides have a tendency 
to fall apart. Driesch afterwards noticed that eggs of the sea-urchin 
treated with sea-water which is free from lime have a tendency to give 
rise to twins. The writer has recently found that twins can be pro- 
duced not only by the absence of lime, but also through the absence of 
sodium or of potassium ; in other words, through the absence of one 
or two of the three important metals in the sea-water. There is, how- 
ever, a second condition, namely, that the solution used for the produc- 
tion of twins must have a neutral or at least not an alkaline reaction. 
The procedure for the production of twins in the sea-urchin egg 
consists simply in this:—the eggs are fertilised as usual in normal 
sea-water and then, after repeated washing in a neutral solution of 
sodium chloride (of the concentration of the sea-water), are placed in 
a neutral mixture of potassium chloride and calcium chloride, or of 
sodium chloride and potassium chloride, or of sodium chloride and 
calcium chloride, or of sodium chloride and magnesium chloride. The 
eggs must remain in this solution until half an hour or an hour after 
they have reached the two-cell stage. They are then transferred into 
normal sea-water and allowed to develop. From 50 to 90°/, of the 
eggs of Strongylocentrotus purpuratus treated in this manner may 
develop into twins. These twins may remain separate or grow 
partially together and form double monsters, or heal together so 
completely that only slight or even no imperfections indicate that the 
individual started its career as a pair of twins. It is also possible to 
control the tendency of such twins to grow together by a change in 
the constitution of the sea-water. If we use as a twin-producing solu- 
tion a mixture of sodium, magnesium and potassium chlorides (in the 
proportion in which these salts exist in the sea-water) the tendency of 
the twins to grow together is much more pronounced than if we use 
simply a mixture of sodium chloride and magnesium chloride. 
